This article deals with the reliability analysis and architecture definition of a fault-tolerant electro-mechanical actuator system for unmanned aerial vehicle applications. Starting from the basic layout of the flight control system of a medium altitude long endurance unmanned aerial vehicle, the attention is focused on the fault mode analysis of the single electromechanical actuator system, with the purpose of pointing out the effects of architectural choices on the system reliability. The electro-mechanical actuator system, developed to be a self-monitoring equipment, has three operating modes: normal, fail-operative and fail-safe. Reliability and safety budgets are quantitatively evaluated via fault tree analysis using typical failure rates of system components, and the most critical paths are identified and discussed

This article deals with the development and performance characterisation of model-based health monitoring algorithms for the detection of faults in an electromechanical actuator for unmanned aerial system flight controls. Two real-time executable position-tracking algorithms, based on predictors with different levels of complexity, are developed and compared in terms of false alarm rejection and fault detection capabilities, using a high-fidelity model of the actuator in which different types of faults are injected. The algorithms' performances are evaluated by simulating flight manoeuvres with the actuator in normal operation as well as with relevant faults (motor coil faults, motor magnet degradation, voltage supply decrease). The results demonstrate that an accurate position-tracking monitor allows to obtain a prompt fault detection and fail-safe mode engagement, while more detailed monitoring functions can be used for fault isolation only.

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The current paper deals with the study of the electrical failures in fault-tolerant\ud flight actuators, with particular reference to the short circuits of the servovalve coils. A highfidelity\ud model of the servovalve of a modern fly-by-wire actuator is developed and validated\ud through experiments, focusing attention on the characterization of the component dynamics in\ud case of partial and total short circuits of the direct-drive motor coils. The servovalve model is\ud then used to simulate a typical on-ground built-in-test procedure to determine the limit\ud condition for the detection of a partial short circuit. Finally, once different possible\ud combinations of short circuits are injected, the degradation of performances of the whole\ud actuator is characterized through experiments, and the servovalve model is used to justify the\ud test results, highlighting and discussing the effects of the failures on the system dynamics